The PCT publication WO 8503707 (hereinafter referred to as the '707 application, equivalent of which is the U.S. Pat. No. 4,659,716) claims desloratadine and exemplifies the process of its preparation by alkaline hydrolysis of loratadine (formula 2, scheme 1), followed by treatment with acetic acid to give the acetate salt of desloratadine, and then converting the acetate into the free base of desloratadine. However, the '707 application does not disclose the purity levels of desloratadine or its impurity profile.

The PCT publication WO 9901450 (equivalent of which is the U.S. Pat. No. 6,506,767) discloses the polymorphic forms 1 and 2 of desloratadine. Preparation of the polymorphic forms 1 and 2 is disclosed by employing alkaline hydrolysis of loratadine by a process similar to that described in the '707 application, and then crystallization from methyl isobutyl ketone, hexanol, methanol, 3-methyl-1-butanol, cyclohexanol, chlorinated solvents such as dichloromethane, ethyl acetate and ether solvents such as dioxane, di-isopropylether, di-n-butylether. However, WO 9901450 too does not disclose the purity levels of desloratadine or its impurity profile.
The PCT application WO 02/42290 claims acid addition salts of desloratadine namely, monoacid, hemiacid and diacid salts. It discloses a process for preparation of diacid salts by reacting loratadine with concentrated mineral acids. It also teaches a process for conversion of diacid salts to monoacid or hemiacid salts by treatment with a solution of a base. It provides new desloratadine hemisulfate salt which is prepared from desloratadine disulfate salt with High Performance Liquid Chromatography (referred to as HPLC herein) purity greater than 99.5% by treatment with a solution of aqueous ammonia.
We have found that desloratadine when prepared according to the prior art processes shows a HPLC peak for an impurity at a relative retention time in the range from about 0.85 to about 0.99 (with respect to desloratadine), which is greater than the discard limit set at less than 0.025% of the total area, when tested according to an HPLC method performed using a Hypersil BDS C8 column (15 cm×4.6 mm, 5 μm particle size) with the following parameters:
Mobile phase:Buffer solution having a pH of about 3, methanol andacetonitrile in a volume ratio of 8:1:1.Injection volume:20 μlFlow rate:1.5 ml/minRun time:75 mins.Discard limit:Set at less than 0.025% of total area